Also doesn't allow specifying initial state as a state vector, e.g. a vector of shape (2**len(qubits),):

>>> sim = cirq.DensityMatrixSimulator()
>>> circuit = cirq.Circuit(cirq.measure(cirq.LineQubit(0)))
>>> sim.simulate(circuit, initial_state=np.array([1, 0]))
...
ValueError: cannot reshape array of size 2 into shape (2,2)

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Input In [20], in <cell line: 1>()
----> 1 sim.simulate(circuit, initial_state=np.array([1, 0]))

File ~/.virtualenvs/pyle/lib/python3.8/site-packages/cirq/sim/simulator.py:518, in SimulatesFinalState.simulate(self, program, param_resolver, qubit_order, initial_state)
    493 def simulate(
    494     self,
    495     program: 'cirq.AbstractCircuit',
   (...)
    498     initial_state: Any = None,
    499 ) -> TSimulationTrialResult:
    500     """Simulates the supplied Circuit.
    501 
    502     This method returns a result which allows access to the entire
   (...)
    516         SimulationTrialResults for the simulation. Includes the final state.
    517     """
--> 518     return self.simulate_sweep(
    519         program, study.ParamResolver(param_resolver), qubit_order, initial_state
    520     )[0]

File ~/.virtualenvs/pyle/lib/python3.8/site-packages/cirq/sim/simulator.py:533, in SimulatesFinalState.simulate_sweep(self, program, params, qubit_order, initial_state)
    522 def simulate_sweep(
    523     self,
    524     program: 'cirq.AbstractCircuit',
   (...)
    527     initial_state: Any = None,
    528 ) -> List[TSimulationTrialResult]:
    529     """Wraps computed states in a list.
    530 
    531     Prefer overriding `simulate_sweep_iter`.
    532     """
--> 533     return list(self.simulate_sweep_iter(program, params, qubit_order, initial_state))

File ~/.virtualenvs/pyle/lib/python3.8/site-packages/cirq/sim/simulator_base.py:345, in SimulatorBase.simulate_sweep_iter(self, program, params, qubit_order, initial_state)
    343 qubits = ops.QubitOrder.as_qubit_order(qubit_order).order_for(program.all_qubits())
    344 initial_state = 0 if initial_state is None else initial_state
--> 345 sim_state = self._create_act_on_args(initial_state, qubits)
    346 prefix, suffix = (
    347     split_into_matching_protocol_then_general(program, sweep_prefixable)
    348     if self._can_be_in_run_prefix(self.noise)
    349     else (program[0:0], program)
    350 )
    351 step_result = None

File ~/.virtualenvs/pyle/lib/python3.8/site-packages/cirq/sim/simulator_base.py:380, in SimulatorBase._create_act_on_args(self, initial_state, qubits)
    378         initial_state = int(initial_state / q.dimension)
    379 else:
--> 380     args = self._create_partial_act_on_args(
    381         initial_state=initial_state,
    382         qubits=qubits,
    383         classical_data=classical_data,
    384     )
    385     for q in qubits:
    386         args_map[q] = args

File ~/.virtualenvs/pyle/lib/python3.8/site-packages/cirq/sim/density_matrix_simulator.py:221, in DensityMatrixSimulator._create_partial_act_on_args(self, initial_state, qubits, classical_data)
    212 if self._ignore_measurement_results:
    213     return act_on_density_matrix_args.ActOnDensityMatrixArgs(
    214         qubits=qubits,
    215         prng=self._prng,
   (...)
    219         dtype=self._dtype,
    220     )
--> 221 return act_on_density_matrix_args.ActOnDensityMatrixArgs(
    222     qubits=qubits,
    223     prng=self._prng,
    224     classical_data=classical_data,
    225     initial_state=initial_state,
    226     dtype=self._dtype,
    227 )

File ~/.virtualenvs/pyle/lib/python3.8/site-packages/cirq/_compat.py:330, in deprecated_parameter.<locals>.decorator.<locals>.decorated_func(*args, **kwargs)
    322     qualname = func.__qualname__ if func_name is None else func_name
    323     _warn_or_error(
    324         f'The {parameter_desc} parameter of {qualname} was '
    325         f'used but is deprecated.\n'
    326         f'It will be removed in cirq {deadline}.\n'
    327         f'{fix}\n',
    328     )
--> 330 return func(*args, **kwargs)

File ~/.virtualenvs/pyle/lib/python3.8/site-packages/cirq/_compat.py:330, in deprecated_parameter.<locals>.decorator.<locals>.decorated_func(*args, **kwargs)
    322     qualname = func.__qualname__ if func_name is None else func_name
    323     _warn_or_error(
    324         f'The {parameter_desc} parameter of {qualname} was '
    325         f'used but is deprecated.\n'
    326         f'It will be removed in cirq {deadline}.\n'
    327         f'{fix}\n',
    328     )
--> 330 return func(*args, **kwargs)

File ~/.virtualenvs/pyle/lib/python3.8/site-packages/cirq/_compat.py:330, in deprecated_parameter.<locals>.decorator.<locals>.decorated_func(*args, **kwargs)
    322     qualname = func.__qualname__ if func_name is None else func_name
    323     _warn_or_error(
    324         f'The {parameter_desc} parameter of {qualname} was '
    325         f'used but is deprecated.\n'
    326         f'It will be removed in cirq {deadline}.\n'
    327         f'{fix}\n',
    328     )
--> 330 return func(*args, **kwargs)

File ~/.virtualenvs/pyle/lib/python3.8/site-packages/cirq/sim/act_on_density_matrix_args.py:307, in ActOnDensityMatrixArgs.__init__(self, target_tensor, available_buffer, qid_shape, prng, log_of_measurement_results, qubits, ignore_measurement_results, initial_state, dtype, classical_data)
    242 @_compat.deprecated_parameter(
    243     deadline='v0.15',
    244     fix='Use classical_data.',
   (...)
    271     classical_data: Optional['cirq.ClassicalDataStore'] = None,
    272 ):
    273     """Inits ActOnDensityMatrixArgs.
    274 
    275     Args:
   (...)
    305             and `qid_shape` is not provided.
    306     """
--> 307     state = _BufferedDensityMatrix.create(
    308         initial_state=target_tensor if target_tensor is not None else initial_state,
    309         qid_shape=tuple(q.dimension for q in qubits) if qubits is not None else None,
    310         dtype=dtype,
    311         buffer=available_buffer,
    312     )
    313     if ignore_measurement_results:
    314         super().__init__(
    315             state=state,
    316             prng=prng,
   (...)
    320             classical_data=classical_data,
    321         )

File ~/.virtualenvs/pyle/lib/python3.8/site-packages/cirq/sim/act_on_density_matrix_args.py:82, in _BufferedDensityMatrix.create(cls, initial_state, qid_shape, dtype, buffer)
     80 else:
     81     if qid_shape is not None:
---> 82         density_matrix = initial_state.reshape(qid_shape * 2)
     83     else:
     84         density_matrix = initial_state

ValueError: cannot reshape array of size 2 into shape (2,2)

Density matrix simulator doesn't accept array with shape=(2,)*2**len(qubits) as an initial state #3958

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